There can be many good reasons for using RAID. A few are; the ability
to combine several physical disks into one larger "virtual" device,
performance improvements, and redundancy.

It is, however, very important to understand that RAID is not a
substitute for good backups. Some RAID levels will make your systems
immune to data loss from single-disk failures, but RAID will not allow
you to recover from an accidental "rm -rf /". RAID will also
not help you preserve your data if the server holding the RAID itself
is lost in one way or the other (theft, flooding, earthquake, Martian
invasion etc.)

RAID will generally allow you to keep systems up and running, in case
of common hardware problems (single disk failure). It is not
in itself a complete data safety solution. This is very important to
realize.

Linux RAID can work on most block devices. It doesn't matter whether
you use IDE or SCSI devices, or a mixture. Some people have also used
the Network Block Device (NBD) with more or less success.

Since a Linux Software RAID device is itself a block device, the above
implies that you can actually create a RAID of other RAID
devices. This in turn makes it possible to support RAID-10
(RAID-0 of multiple RAID-1 devices), simply by using the RAID-0 and
RAID-1 functionality together. Other more exotic configurations, such
a RAID-5 over RAID-5 "matrix" configurations are equally
supported.

The RAID layer has absolutely nothing to do with the filesystem
layer. You can put any filesystem on a RAID device, just like any
other block device.

Often RAID is employed as a solution to performance problems. While
RAID can indeed often be the solution you are looking for, it is not a
silver bullet. There can be many reasons for performance problems, and
RAID is only the solution to a few of them.

See Chapter one for a mention of the performance characteristics of
each level.

This setup lets the machine swap in parallel on seven SCSI devices. No
need for RAID, since this has been a kernel feature for a long time.

Another reason to use RAID for swap is high availability. If you set
up a system to boot on eg. a RAID-1 device, the system should be able
to survive a disk crash. But if the system has been swapping on the
now faulty device, you will for sure be going down. Swapping on a
RAID-1 device would solve this problem.

There has been a lot of discussion about whether swap was stable on
RAID devices. This is a continuing debate, because it depends highly
on other aspects of the kernel as well. As of this writing, it seems
that swapping on RAID should be perfectly stable, you should however
stress-test the system yourself until you are satisfied with the
stability.

You can set up RAID in a swap file on a filesystem on your RAID
device, or you can set up a RAID device as a swap partition, as you
see fit. As usual, the RAID device is just a block device.

The classic raidtools are the standard software RAID management
tool for Linux, so using mdadm is not a must.

However, if you find raidtools cumbersome or limited, mdadm (multiple
devices admin) is an extremely useful tool for running RAID systems.
It can be used as a replacement for the raidtools, or as a supplement.

The mdadm tool, written by
Neil Brown, a software engineer at the University of
New South Wales and a kernel developer, is now at version 1.4.0 and
has proved to be quite stable. There is much positive response on the
Linux-raid mailing list and mdadm is likely to become widespread in the
future.